The present invention relates to plate-like barium ferrite particles for use in magnetic recording and a process for producing the same, and more in detail, the present invention relates to the plate-like barium ferrite particles (BaO.6Fe.sub.2 O.sub.3) having a specific surface area of 11 to 50 m.sup.2 /g, a value of magnetization of higher than 25 emu/g in a magnetic field of 10 KOe and a coercive force of 600 to 1100 Oe, and the process for producing the plate-like barium ferrite particles, comprising the steps of autoclaving an aqueous alkaline suspension of pH of higher than 11 containing Fe(III) and Ba ions at an atomic ratio of 6:1 to 8:1 at a temperature of 110.degree. to 190.degree. C., to form fine particles of 2BaO.9Fe.sub.2 O.sub.3, and autoclaving the aqueous suspension containing the thus formed 2BaO.9Fe.sub.2 O3 at a temperature of 280.degree. to 320.degree. C.
In recent years, non-acicular ferromagnetic particles of a favorable dispersibility provided with a suitable coercive force(Hc) have been required as the magnetic material for recording, particularly as the magnetic material for the perpendicular magnetic recording. In general, as the non-acicular ferromagnetic particles, those of barium ferrite have been well known.
However, because of the excessively high coercive force thereof of higher than 3000 Oe in general, non-acicular ferromagnetic particles of barium ferrite are not favorable as the magnetic material for magnetic recording.
Accordingly, as a method for reducing the coercive force of the particles of barium ferrite, for instance, a process in which a part of Fe(III) of barium ferrite is substituted by other metal ions such as Co(II) and Ti(IV), thereby obtaining the magnetic material with a reduced coercive force while retaining the change of the saturation magnetization in a state as small as possible has been tried.
In addition, since the average size of the particles of barium ferrite are a few micrometers, at least about 1 micrometer, the dispersibility thereof in the case of preparation of a magnetic paint therefrom is poor, and such particles are not favorable as the magnetic material for magnetic recording.
Namely, as a magnetic material for magnetic recording, those of the particle size of as small as 0.05 to 0.5 micrometer have been demanded. The fact is clearly seen in Japanese Patent Application Laying-Open No. 53-20596 (1978) from the disclosure of "there is a difficulty in uniformly coating in the case where the particle size is over 0.5 micrometer . . .".
On the other hand, as a process for producing barium ferrite, a method of autoclaving an aqueous alkaline suspension containing Ba ions and Fe(III)(hereinafter simply referred to as the autoclaving method) has been known, and according to the autoclaving method, the particles of barium ferrite appear in the aqueous suspension as a precipitate. The thus precipitated particles exhibit ordinarily hexagonal plate-like shapes and their magnetic properties depend on the particles' size distribution and the average particle size which in turn depend on the conditions of their formation.
As a result of the present inventors' studies on the relationship between the conditions of formation of the barium ferrite particles, such as the ratio of Fe(III) to Ba ions, the concentration of the aqueous alkaline solution, the reaction temperature and the reaction time in the above-mentioned autoclaving method and the average particle size, the distribution of the particle size and the magnetic properties of the thus obtained barium ferrite particles in order to obtain the plate-like barium ferrite particles of an average particle size of about 0.05 to 0.5 micrometer and a very small coercive force with a value of magnetization(M) not remarkably reduced, the present invention has been attained.